In order to minimize problems arising from electrostatic charging during the manufacture and use of imaging elements various well known methods can be used to introduce an electrically-conductive layer into an imaging element to dissipate accumulated static charge. In the case of a photographic element, the electrically-conductive layer can be a subbing layer, an intermediate layer, and especially an outermost layer either overlying a silver halide emulsion layer or a backing layer on the opposite side of the support from the silver halide emulsion layer(s). For typical polyolefin-coated photographic papers, this electrically-conductive layer is applied to the support as an antistatic backing layer. In addition to providing suitable charge dissipation properties, such backing layers must also provide the ability to receive printed information (e.g., bar codes, alphanumeric data or other types of indicia or identification) typically applied by means of dot matrix or inkjet printers as well as the ability to retain both antistatic and print-retaining properties after the paper has been subjected to photographic processing (viz., "backmark retention"). Further, such conductive print-retaining backing layers also must exhibit suitable physical properties in order to enable heat or tape splicing and to minimize dusting or trackoff during conveyance. Heat splicing of rolls of photographic paper is often performed during printing operations and requires sufficient mechanical strength to resist peeling as the web is conveyed through automatic high speed photographic processing equipment. Failure caused by poor splice strength can result in poor conveyance of the web leading to jamming of the processing equipment. In addition, trackoff or pick off of material from the backing layer during conveyance can result in an undesirable build-up of particulate debris on conveyance rollers and other contact surfaces which can produce surface defects in the product.
A wide variety of conductive antistatic agents can be incorporated in antistatic layers to produce a broad range of surface electrical conductivities. Many of the traditional antistatic layers used in photographic elements employ materials which exhibit predominantly ionic conductivity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sols, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like have been taught in prior art. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of the surrounding environment. At low relative humidities and temperatures, the diffusional mobilities of the charge-carrying ions are greatly reduced and the bulk conductivity is substantially decreased. Because of the aqueous solubility and ion-exchange properties of such materials, unprotected antistatic layers containing ionic conductors typically do not exhibit antistatic properties after photographic processing.
U.S. Pat. No. 3,525,621 discloses that antistatic properties can be provided for photographic paper by applying a layer containing an aqueous colloidal silica sol, preferably one consisting of silica particles having a specific surface area of about 200-235 m.sup.2 /g, in combination with an alkylaryl polyether sulphonate. However, because of the high solubility of such an alkylaryl polyether sulphonate in aqueous media, it can be leached out during processing resulting in poor backmark retention by the layer. Further, antistatic backing layers for photographic papers containing colloidal silica without a polymeric binder typically exhibit microcracks upon drying which can lower the surface conductivity. Calcium stearate from the paper base can leach out through these microcracks during photographic processing and deposit stearate sludge in the processing tanks, which can require costly clean-up operations. In addition, such colloidal silica-based antistatic backing layers typically exhibit poor backmark retention.
The use of synthetic hectorite clay particles as an antistatic additive to a silica-containing antistatic layer for photographic paper is taught in U.S. Pat. No. 4,173,480. However, the hydrophilicity and other surface properties of the synthetic hectorite clay results in poor backmark retention.
Writeable, antistatic backcoatings for polyolefin-coated photographic paper containing one or more water-soluble or latex polymeric binders in combination with a cation-modified colloidal silica and an alkali salt were disclosed in U.S. Pat. Nos. 4,705,746; 4,895,792; and 5,045,394. For, example, the backcoatings of U.S. Pat. No. 5,045,394 were disclosed to exhibit good printability, writeability, minimal dyestain in developing solutions, good tape adhesion, and adequate antistatic characteristics. However, antistatic properties of such backcoatings typically are lost after photographic processing.
A print-retaining layer for polyolefin resin-coated photographic paper containing a granular tooth-providing ingredient in a polymeric binder consisting of an addition product of an alkyl methacrylate, an alkali salt of an ethylenically unsaturated sulfonic or carboxylic acid, a vinyl benzene monomer, and an ethylenically unsaturated crosslinking agent is disclosed in U.S. Pat. Nos. 5,075,164 and 5,405,907. Although the print-retaining layer provides adequate backmark retention for use with most automatic processors, the layer can be damaged or entirely removed, resulting in poor backmark retention, when passed through automatic processors having harsher operating conditions. Also, photographic elements having such print-retaining layers can exhibit various deficiencies, such as blocking, incompatibility of ingredients, and pickoff during manufacture.
U.S. Pat. Nos. 5,244,728 and 5,385,968 disclose the use of aqueous coating formulations containing alumina-modified colloidal silica in combination with an antistatic agent consisting of a non-ionic polymer in conjunction with an alkali metal salt, for example, polyethylene ether glycol and lithium nitrate, and a polymeric binder consisting of an addition product of alkyl methacrylate, an alkali metal salt, and a vinyl benzene to prepare antistatic backing layers for photographic paper. Although these backing layers provide adequate backmark retention properties, antistatic protection is provided only before processing since the metal salts used as antistatic agents are soluble in processing solutions. Further such backing layers are unsuitable because they lack sufficient mechanical integrity as demonstrated by poor spliceability and excessive track off properties.
An antistatic layer for use on polyolefin resin-coated photographic paper containing a polymeric latex binder and a non-ionic surface-active compound including poly(ethylene oxide) and alkali metal salt wherein the non-ionic surface-active compound is present at between 0.1 and 4 percent of the dry weight of the antistatic layer is disclosed in U.S. Pat. No. 5,683,862. Such antistatic layers exhibit improved spliceability and trackoff properties as well as acceptable post-processing backmark retention. However, the antistatic properties of such layers are substantially diminished after photographic processing.
Antistatic layers containing electronic conductors such as conjugated conductive polymers, conductive carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconductive thin films can be used more effectively than ionic conductors to dissipate static charge since their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Of the various types of electronic conductors, electrically-conductive metal-containing particles, such as semiconductive metal oxides, are particularly effective when dispersed with suitable polymeric film-forming binders in combination with polymeric non-film-forming particles as described in U.S. Pat. Nos. 5,340,676; 5,466,567; 5,700,623. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example: U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; 5,484,694; and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include Sb-doped tin oxide, F-doped tin oxide, Al-doped zinc oxide, and Nb-doped titania. Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other conductive metal-containing granular particles including metal borides, carbides, nitrides, and suicides have been disclosed in Japanese Kokai No. JP 04-055,492.
One serious deficiency of such electronic conductor materials is that, especially in the case of semiconductive metal-containing particles, the particles typically are intensely colored (i.e., black, gray, blue, green, yellow, etc.) which renders them unsuitable for use in coated layers on many photographic paper supports, particularly at high weight loadings or dry weight coverages. However, composite conductive particles consisting of a thin layer of conductive metal-containing particles deposited onto the surface of non-conductive core particles can exhibit much lighter color while retaining much of the bulk conductivity of homogeneous conductive metal-containing particles described hereinabove. For example, composite conductive particles consisting of two dimensional networks of fine antimony-doped tin oxide crystallites in association with amorphous silica deposited on the surface of much larger, non-conductive metal oxide particles (e.g., silica, titania, etc.) and a method for their preparation are disclosed in U.S. Pat. Nos. 5,350,448; 5,585,037; and 5,628,932. Other suitable composite conductive granular particles include titanium dioxide particles having a uniform and strongly adherent thin layer of antimony or fluorine-doped tin oxide formed on their surfaces as disclosed in U.S. Pat. Nos. 4,373,013 and 4,452,830; inorganic titanate particles (e.g., barium titanate, strontium titanate, calcium titanate, magnesium titanate, etc.) with coatings of antimony-doped tin oxide and dense amorphous silica as disclosed in British Patent No. 2,253,839; iron oxide particles coated with antimony-doped tin oxide particles as disclosed in U.S. Pat. No. 4,917,952; granular barium sulfate particles coated with a thin layer of antimony or fluorine-doped tin oxide as disclosed in U.S. Pat. No. 5,585,037; and platelet-like or lamellar non-conductive particles (e.g., talc, mica, kaolinite, bentonite, montmorillonite, smectite clay, hematite) coated with a thin conductive layer of antimony-doped tin oxide as described in U.S. Pat. Nos. 4,568,609; 4,917,952; 5,322,561; 5,472,640; 5,585,037; and 5,677,039; for example. Such composite conductive particles are disclosed in U.S. Pat. Nos. 4,373,013; 5,466,536; 5,488,461; and British Patent No. 2,253,839 to be useful in conductive backing layers on photographic, electrographic, and electrophotographic support materials, especially resin-coated photographic paper. Also, fibrous or needle-like conductive materials including composite conductive particles consisting of acicular or fibrous nonconductive metal oxide core particles, such as TiO.sub.2, K.sub.2 Ti.sub.6 O.sub.13, 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3, 2Al.sub.2 O.sub.3. B.sub.2 O.sub.3 or 2MgO.B.sub.2 O.sub.5 coated with a thin conductive layer of antimony-doped tin oxide or zinc oxide have been described in U.S. Pat. Nos. 4,880,703; 5,273,822; and Japanese Kokai Nos. Sho 59-10280, 61-26933, and 62-59528. Such acicular or fibrous composite conductive particles can be used in conductive backing layers for photographic papers as disclosed in U.S. Pat. No. 5,466,536; European Patent Application No. 616,252; and Japanese Kokai Nos. Sho 01-262537.
However, there is difficulty in the preparation of conductive backings containing composite conductive particles, in that the dispersion of composite conductive particles of the type described hereinabove, especially acicular or fibrous composite conductive particles, in an aqueous vehicle using conventional wet milling dispersion techniques and traditional ceramic or steel milling media is well known to be very difficult to accomplish without abrading the thin conductive layer from the core particle or shattering the high aspect ratio acicular core particles into lower aspect ratio fragments. Fragile composite conductive particles often cannot be dispersed effectively because of limitations on milling intensity and duration dictated by the need to minimize degradation of the morphology and electrical properties as well as the introduction of attrition-related contamination from the dispersion process. The presence of colloidal ceramic particles from the attrition of conventional ceramic media can promote destabilization of the dispersion leading to heteroflocculation. Further, as described in U.S. Pat. No. 5,480,752, the use of conventional dispersing agents in the dispersion process can produce unstable dispersions which flocculate and settle rapidly. The use of such low stability dispersions to prepare conductive layers for imaging elements can produce nonhomogeneities in the coated layers leading to decreased conductivity and increased optical losses due to scattering by agglomerates or aggregates of particles.
The preparation of dispersions of submicron-size particles of crystalline materials useful in imaging elements (e.g., filter dyes, sensitizing dyes, image-forming couplers, antifoggants, etc.) by comminution using conventional wet milling techniques well known in the pigment and paint industry, such as high-speed mixing, ball or roller milling or high energy media milling, has been disclosed in U.S. Pat. Nos. 4,940,654; European Application No. 649,858 and Japanese Examined Application No. 02-601,887. Such comminution processes involve physical attrition of the material useful in imaging elements using milling media generally selected from a variety of dense, hard materials, such as steel, ceramic or glass. The action of such milling media on the particulate materials results in particle size reduction as well as dispersion. The resulting fine particle dispersions can be stabilized using a variety of surfactants or dispersing aids to prevent re-agglomeration of the dispersed fine particles. It also may be necessary to adjust pH during the milling process to stabilize the dispersion. The milling energy input levels required to produce dispersions of very small size particles can result in the generation of excessive amounts of attrition-related contamination from erosion of the milling media and wear of components of the milling equipment. Such attrition-related contamination is usually present in the form of dissolved species or particulates of sizes comparable to or larger than those of the dispersed particles and can lead to both physical and sensitometric defects in imaging elements containing these contaminated dispersions. Further, heat generated during high intensity milling operations may initiate chemical reactions, introduce surface defects or promote phase changes in the material being dispersed. Thus, the physical, chemical, electrical, optical, and mechanical properties of the dispersed particles may differ substantially from those of the particulate materials before milling. Such variations in properties can adversely affect the performance of imaging elements including layers prepared from dispersions of these materials.
A method for preparing dispersions of submicron-size particles of crystalline materials useful in imaging elements by a wet milling process using small milling media consisting of fine polymeric resin beads that results in reduced levels of attrition-related contamination in the dispersions has been disclosed in U.S. Pat. Nos. 5,478,705; 5,500,331; 5,513,803; and 5,662,279. Such polymeric milling media consist essentially of a polymeric resin and are nominally spherical in shape, chemically and physically inert, substantially free of metals, solvents or monomers, and are sufficiently hard to avoid being chipped or crushed during the dispersion process. Suitable polymeric milling media are typically less than about 250 .mu.m in diameter. Compounds useful in imaging elements that can be dispersed using polymeric milling media are claimed in U.S. Pat. No. 5,478,705 and include dye-forming couplers, development inhibitor release couplers, development inhibitor anchimeric release couplers, masking couplers, filter dyes, thermal transfer dyes, optical brighteners, nucleators, development accelerators, oxidized development scavengers, ultraviolet radiation absorbing compounds, sensitizing dyes, development inhibitors, antifoggants, bleach accelerators, magnetic particles, lubricants, and matting agents. Further, polymeric milling media can be used to prepare aqueous dispersions of submicron size particles of organic pigments suitable for use in inkjet inks, as disclosed in U.S. Pat. Nos. 5,651,813 and 5,679,138. However, the utility of polymeric milling media for dispersing electrically-conductive metal-containing composite particles, for inclusion in antistatic backing layers or electrodes for imaging elements has been neither disclosed nor contemplated in prior art.
Thus, one object of the present invention is to provide an effective antistatic backing layer on an imaging element having an opaque or translucent support, particularly a polyolefin resin-coated photographic paper, with suitable backmark receiving and retention properties. The present invention finds particular utility in the photofinishing industry by enabling the printing of alphanumeric characters, barcodes, indicia or other markings onto the back of photographic paper prints using dot matrix or inkjet printers, for example. Photofinishing requirements are particularly stringent because such backing layers must survive photographic processing by automatic processing equipment under harsh conditions in order to be useful. Another object of the present invention is to provide an electrically-conductive layer on a surface to which print or ink markings can be applied, for example by inkjet or thermal printing, wherein the original surface does not possess the desired wetting, spreading, drying or adhesion properties necessary to ensure the clarity and permanence of the applied markings.
Because the requirements for electrically-conductive backing layers to be useful in a photographic element are extremely demanding, the art has long sought to develop improved conductive layers exhibiting a balance of the necessary chemical, physical, optical, and electrical properties. As indicated hereinabove, the prior art for providing electrically-conductive layers useful for photographic elements is extensive and a wide variety of suitable electroconductive materials have been disclosed. However, there is still a critical need in the art for improved conductive backing layers for photographic papers which can be manufactured at a reasonable cost, which are resistant to the effects of humidity change, which are durable and abrasion-resistant, which are transparent or translucent, which are colorless or lightly colored, which do not exhibit adverse sensitometric or photographic effects, which exhibit acceptable adhesion to the support or an underlying layer, which exhibit suitable cohesion, acceptable heat spliceability, good trackoff characteristics, and which retain their antistatic properties and backmark receiving and retention properties after photographic processing.
It is toward the objective of providing conductive backing layers for imaging elements, especially polyolefin-coated photographic paper, having composite metal-containing conductive particles dispersed in a polymeric film-forming binder without causing degradation of either the physical or electrical properties of the composite conductive particles or introducing attrition-related contamination during the dispersion process that more effectively meet the diverse needs of such imaging elements than those of the prior art that the present invention is directed.